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Re: [lvm-devel] [PATCH] Fix RHBZ 754198 (multiple dmeventd snapshot extensions)



Dne 16.11.2011 10:30, Petr Rockai napsal(a):
Alasdair G Kergon<agk redhat com>  writes:
- real problem is the *calculation* must be going wrong if one
extend is insufficient to solve the problem!

No, the real problem is that the snapshot is being filled and reaches
the threshold again, after first extension is done. Nothing wrong with
calculation.

Calculation should happen per every 5% increase.

Yet the logic there is IMHO not the best abd I'd say there are couple things to improve.

Recalc is made on every 5% increase and 10 second delay is relatively
way too long time - when you could easily write over 100MB/s these days even
on very low level laptops - and better hardware may easily reach over 400MB/s

(We already have reports from users, which needed to use very low watermark
level to catch fast writing apps overfilling snapshot)

I'm playing with something similar on thinp support,
and I think we need something smarted here - which works with relative sizes
(well thinp will sleep at least - why not making the same for snapshots ?)

Anyway - related to Petr's patch - I don't like value '6' -
I would put there i.e. 128 - so we could keep real errors sequential within 7 bits ? and non-real-errors either 0 or > 127 (will we need more of these ?)


Zdenek


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